The efficient coupling of light from optical fibers to waveguides extending onto optical chips is of primary importance in various fields, including optical communications. Several techniques are known to couple optical fibers to on-chip optical waveguides. In typical approaches, the protective coating of the optical fiber is removed along a small length, and the bare portion of optical fiber cleaved and either glued directly to the optical chip, or inserted in a glass or ceramic ferule which is itself polished and glued to the optical chip. Such techniques can be used in non-hermetic environments since there is no air gap between the optical fiber and the chip. Other approaches make use of various type of lenses to couple the light into the on-chip optical waveguide, such as for example shaped-tip fiber, aspheric lenses, GRIN lenses, etc. However, such approach can typically be used only in hermetic environments. In either case, the subassembly thus realized leaves a piece of coated optical fiber, typically provided with an optical connector on its distal end, dangling from the optical chip, and forms what is called in the field a “pigtailed” component.
Referring to FIG. 1A (PRIOR ART), in some assemblies the optical fiber is attached to the side of a chip. In this case, appropriate on-chip inverted tapers and/or spot size converters can be used to improve the coupling efficiency between the on-chip optical waveguide and the optical fiber, by adapting as much as possible the optical modes on each side. V-grooves patterned in the chip structure can be used to ease the alignment of the fiber in front of the on-chip optical waveguide, ultimately making passive alignment possible. As shown in FIG. 1B (PRIOR ART), the fiber can also be attached to the top surface of the chip, for example using surface grating couplers. In this latter case, the fiber can be polished at appropriate angles such as described in U.S. Pat. No. 8,639,073 (PELLETIER et al).
The coupling of light from optical fibers into an optical chip can present some incompatibilities with typical chip design and manufacturing processes. In many applications, the optical chip includes electrical ports which need to be connected to an external circuit. Those connections can be realized through wire bonding. The electrical ports generally lie near the edges of the chip. However, for increased electrical port densities, it may be more suitable to use connections anywhere through the substrate. Through Silicon Vias (TSV), in the case of silicon-based chips, are good examples of such connections. To provide a high density of electrical connections, surface mount processes such as ball grid arrays (BGA) can be used to realize the permanent joint between the optical chip and the external electronic circuit (or Printed Circuit Board, PCB), as shown in FIG. 2 (PRIOR ART). BGAs or other surface mount processes typically use reflow in an oven to melt the solder balls at temperature above 183° C. for eutectic lead-tin solder, and above 220° C. for lead free tin based solder complying with Restriction of Hazardous Substances Directive (ROHS) such as the SAC305 solder, that contains 96.5% tin, 3% silver, and 0.5% copper. Such high temperatures have the effect of damaging optical fibers having a standard protective coating such as a dual coated acrylate, and damaging other heat sensitive components such as typical low temperature resistance polymer-based fiber connectors. Additionally, the very presence of an optical fiber attached to the chip is incompatible with many automated assembly techniques such as Surface Mount Technology (SMT); the pick-and-place machines used in such assembly lines are not able to manipulate an optical chip having a loose piece of fiber and its connector dangling.
There remains a need for a technique which makes it possible and practical to connect an optical fiber to a waveguide on a chip which ensures a proper alignment of the fiber with low insertion loss, while being compatible with the use of high temperatures in reflow processes.